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United States Patent |
5,028,034
|
Bishop
,   et al.
|
July 2, 1991
|
Continuous feed shaft retort appartus for recovery of non-ferrous metals
Abstract
A method and apparatus for recovering primary metals from pyrometallurgical
process dust, in which a vertical shaft retort, situated in a furnace or
afterburner associated with a pyrometallurgical apparatus, such as an
inclined rotary reduction smelter, and preferably having a tapered
cross-section with the larger end at the bottom, receives greenball
pellets from a pelletizer, vaporizes volatile metal metals therein, and
removes them to an associated volatile metal recovery apparatus, while
reducing and recovering the major metal oxide components in the pellets in
metallized form.
Inventors:
|
Bishop; Norman G. (Ruidoso, NM);
Bottinelli; N. Edward (Dallas, TX);
Kotraba; Norman L. (Tega Cay, SC)
|
Assignee:
|
Zia Patent Company (Dallas, TX)
|
Appl. No.:
|
501160 |
Filed:
|
March 29, 1990 |
Current U.S. Class: |
266/150; 266/154 |
Intern'l Class: |
C21C 005/38 |
Field of Search: |
266/150,154,176,177,175
|
References Cited
U.S. Patent Documents
1508451 | Sep., 1924 | Gray et al. | 266/154.
|
1961425 | Jun., 1934 | Maier | 266/154.
|
2433615 | Dec., 1947 | Mahler | 266/154.
|
2457552 | Dec., 1948 | Handwerk et al. | 266/150.
|
2492438 | Dec., 1949 | Pierce | 266/150.
|
Primary Examiner: Kastler; S.
Attorney, Agent or Firm: Dougherty; Ralph H.
Claims
We claim:
1. Apparatus for recovering primary metals from pyrometallurgical process
dust comprising:
means for pelletizing said dust with solid carbonaceous material and a
binder to form pellets;
means for drying said pellets to form greenball pellets;
a vertical shaft retort, having a tapered cross-section, with the larger
cross-section at the bottom end of the retort, said retort having an upper
preheating and coking zone, an intermediate transition zone, and a lower
reduction and vaporization zone;
means at the upper end of said retort for charging said vertical shaft
retort with said greenball pellets and means at the lower end of said
retort for causing them to move continuously downward through said retort;
means for providing heat to the interior of said retort by heating the
exterior thereof;
means in the upper portion of said retort for prereducing and coking the
greenball pellets;
conduit means communicating with the top of said shaft retort for removing
released water vapor and gases;
means in the lower portion of said retort for reducing said pellets and
vaporizing the volatile materials therein to form metallic gas and
released process gas in the reduction and vaporization zone;
extraction means for removing the metallic gas and released process gases
and volatiles from the interior of the retort from the transition zone
intermediate the ends of the retort; and
means for withdrawing reduced pellets from the bottom of said retort.
2. Apparatus according to claim 1, wherein said vertical shaft retort has a
tapered cross-section, with the larger cross-section at the bottom end of
the retort.
3. Apparatus according to claim 1, further comprising means for removing
volatilized metals from the removed gas, and means for recovering said
metals.
4. Apparatus according to claim 1, wherein said pelletizing means includes
sources of dust, carbonaceous material, and binder, a mixer, each source
communicating with said mixer, and an associated pelletizer.
5. Apparatus according to claim 1, wherein said dryer comprises a source of
heat, a preheating chamber, a drying chamber, a continuous moving belt
means movable through both chambers, and means for moving heated gases
upwardly through the belt in one chamber and downwardly through the belt
in the other chamber.
6. Apparatus according to claim 1, wherein said charging means is a holding
bin in barometric sealing relation with said retort.
7. Apparatus according to claim 3, wherein said means for recovering said
volatilized metals comprises a splash condenser.
8. Apparatus according to claim 7, wherein said means for recovering said
volatilized metals further comprises an associated fabric filter.
9. Apparatus according to claim 1, wherein said means for heating said
retort comprises an inclined rotary reduction smelting furnace, and said
retort is situated at least partly within the afterburner of said smelting
furnace.
10. Apparatus according to claim 9, further comprising means for adding
additional heat to the discharge zone of said retort.
11. Apparatus according to claim 10, wherein said means for adding
additional heat to the discharge zone of said retort comprises at least
one oxy-fuel burner, whereby both heat and reducing gas are added to said
discharge zone reducing any remaining oxides in the spent residue.
12. Apparatus according to claim 1, wherein said extraction means for
removing the metallic gas and released process gases and volatiles from
the interior of the retort is an upwardly inclined conduit communicating
with the retort at the transition zone.
Description
BACKGROUND OF THE INVENTION
This is a division of application Ser. No. 07/323,297, filed Mar. 14, 1989.
The subject invention relates to a method and apparatus for recovering
environmentally hazardous metals from flue dust or fly ash which are
generally considered waste byproducts of primary pyrometallurgical
processes. More particularly, it is a continuous feed shaft retort process
for recovery of non-ferrous metals from secondarily oxidized flue dusts
recovered from primary pyrometallurgical metal producing processes. The
invention is particularly adaptable for use with an inclined rotary
reduction smelter or other primary metallurgical process from which energy
is derived to supply the present process, but which energy would otherwise
be wasted. The invention is also operable by using primary energy sources.
The present invention is useful for recovering all of the known "volatile
metals". These include, without limitation, zinc, lead, sodium, potassium,
cadmium, arsenic, mercury, and barium.
The invented apparatus will accomplish certain results with energy savings
not heretofore available to vertical retort processes for recovery of
non-ferrous metals. Previous vertical retort processes such as invented by
E. C. Handwerk et al, in U.S. Pat. No. 2,457,552. were specifically
designed to smelt zinc from high grade (oxide or oxidized) primary zinc
ores. The object of the Handwerk et al patent was to overcome certain
technical and economic problems inherent with previous horizontal and
vertical retort processes. Premature condensing of zinc vapors on the cold
burden as the vapors rose through the unheated extension (top) of the
vertical retort resulted in an excessive recirculating load of zinc within
the retort and hindered the free movement of the burden downward through
the retort; thus decreasing productivity and increasing the operating
costs of the process.
To overcome the "refluxing" (which is the condensing of volatile metal on
the cold burden) actions of the zinc vapor and prematurely condensed
liquid metal on the burden in the top of the retort, the burden had to be
preheated or precoked at temperatures between 850.degree. and 900.degree.
C. before being fed to the vertical retort. Such precoking also served to
remove moisture and volatile matter from the carbon reductant prior to the
burden entering the vertical retort and thereby reducing the evolution of
process limiting H.sub.2 O, and CH4 gases inside the retort. Preheating,
or precoking, actions thus improved the productivity and economics of the
vertical retort process.
Precoking of the burden involves the use of an independent autogenous grate
coker in which a blended burden of oxidized zinc ore, bituminous coal,
anthracite coal and/or coke are burned under oxygen controlled atmospheric
condition to prevent the temperature in the bed of the burden from
exceeding the point at which premature reduction of the zinc oxide to zinc
metal and vapor occurs. By precoking the vertical retort burden material,
sufficient agglomeration strength is achieved to allow the material to
withstand the rigors of passing down through the shaft of the retort
without undergoing excessive disintegration which would cause loss of bed
permeability in the shaft and inhibit the free passage of gases through
the bed.
As the autogenous coking step is a prerequisite for the burden before it
enters the Handwerk et al vertical retort process, certain environmental
problems are inherent. It is known that in the case of smelting electric
arc furnace (EAF) flue dusts, which contain oxides of non-ferrous metals,
reduction and vaporization of the metals begin to occur in the same
temperature range as is employed in the autogenous coker. While the gases
generated in the autogenous coker pass through an afterburning step, waste
gas scrubbing has not been thought to be necessary. Thus, reoxidized fumes
of non-ferrous metals can be emitted with the stack gases from the coker,
creating a potential environmental hazard in the area surrounding the
autogenous coker.
Further, between the autogenous coking step and the Handwerk et al vertical
retort step, the hot coked material must be protected from atmospheric air
to prevent uncontrolled ignition and sintering of the burden. Such
uncontrolled sintering would clinker the burden and release volatilized
metal fumes to the atmosphere.
In Handwerk et al U.S. Pat. No. 2,457,552, a claim is made to introduce
air, or a combustion supporting gas (which implies a possible mixture of
air and oxygen), into the bottom of the retort for the purpose of igniting
with residue carbon in the burden to generate additional heat and
"displacement" gas inside the retort. Careful control is necessary to
prevent sintering or clinkering of the burden in the shaft of the retort
and to avoid generating excessive amounts of diluting gases, especially
process limiting carbon dioxide. By burning the residue carbon in the
spent burden exiting the heated section of the retort, additional heat and
products-of-combustion gas (displacement gas) is generated. The hot
displacement gas transfers the heat upward into the shaft burden, thus
improving the energy efficiency of the process.
The New Jersey Zinc (Handwerk) vertical retort has straight vertical sides
whereas the retort of the present invention has inclined sides with a
larger cross-section at the bottom than at the top. The New Jersey Zinc
vertical retort requires coke briquets and processes only high grade
material and takes about twenty-four hours to treat the material. The
invented apparatus utilizes only pellets which need only be dried and not
pre-coked, processes any type of metallized oxides, particularly low grade
materials, and does so in a considerably shorter time, averaging only
about four hours residence time of the material within the retort.
OBJECTS OF THE INVENTION
It is a principal object of this invention to provide a method and
apparatus for recovering metal values from metallurgical process dusts
such as EAF flue dusts.
Another object of the present invention is to provide a system for recovery
of metals from flue dusts, fly ash, and other waste products of
pyrometallurgical processes, which avoids the requirement for sintered or
coked feed stock.
Another object of the present invention is to provide a system for recovery
of metals which utilizes a stoichiometrically balanced and pelletized feed
material which is cured and reduced in one continuous operation.
It is also an object of the present invention to provide a system for
recovery of metals including forming greenball pellet feed materials,
which have excellent thermal shock resistance and adequate physical
strength for direct placement into the high temperature atmosphere of the
shaft retort without suffering critical disintegration.
It is another object of this invention to provide a means for processing
secondary flue dusts from metallurgical processes.
It is another object of this invention to provide a process for recovery of
zinc and other volatile metals from pyrometallurgical dusts.
It is also an object of this invention to provide an energy efficient means
for operating a shaft retort.
It is another object of this invention to provide a shaft retort process
which requires substantially less fixed carbon in the solid burden feed
than was heretofore required.
It is also an object of the present invention to provide a process for
recovery of metals from waste products which process is capable of
handling low as well as high metallic content (ferrous and non-ferrous)
feed material.
It is also an object of the present invention to provide a system for
recovery of metals from waste products which has greatly improved
gas-to-solids heat transfer within the pellet feed material.
It is also an object of the present invention to provide a retort system
for recovery of metals which has significant improvement in the
productivity of the shaft retort, requiring less relative working volume
(cubic feet capacity) per ton of material processed per unit of time.
BRIEF DESCRIPTION OF THE DRAWINGS
The foregoing and other objects will become more readily apparent by
referring to the following detailed description and the appended drawings,
in which:
FIG. 1 is a schematic flow sheet of the apparatus of the subject invention
including a shaft retort and affiliated gas handling and pellet handling
equipment.
FIG. 2 is a schematic diagram of the prior art inclined rotary reduction
smelter (IRRS) process.
FIG. 3 is a schematic diagram of the present invention in conjunction with
an inclined rotary reduction smelter.
FIG. 4 is a schematic diagram of the invented apparatus according to FIG. 1
showing an alternative pellet treatment at the discharge end of the retort
.
DETAILED DESCRIPTION OF THE INVENTION
The present invention as described herein will smelt and. recover
environmentally hazardous metals from flue dust or fly ash which are
generated as waste by-products by primary pyrometallurgical processes.
Referring now to the drawings, and particularly to FIG. 1, the invented
apparatus for recovering primary metals from pyrometallurgical process
dust features a vertical shaft retort 10, preferably having a tapered
cross-section, with the larger cross-section at the bottom end of the
retort, and having charging means at its upper end for charging the
vertical shaft retort with greenball pellets, means for removing or
withdrawing the reduced pellets from the bottom of the retort, and causing
the pellets to move continuously downward through the retort. The retort
10 is situated in a furnace 12, which is preferably an afterburner from a
pyrometallurgical apparatus, such as an inclined rotary reduction smelter.
The afterburner heats the exterior of the retort, which transfers heat to
the interior of the retort, which is divided into three operating zones, a
preheat and coking zone, a transition zone, and a reduction and
vaporization (or volatilization zone.
Ahead of the retort, and communicating therewith, are a pelletizer which
mixes process dust with solid carbonaceous material and a binder to form
pellets, and a dryer for drying the pellets to form greenball pellets for
charging to the retort.
A volatilized metal duct communicates with the interior of the retort and a
splash condenser to conduct spent process gas and volatilized metals to
the splash condenser.
The continuous feed shaft retort receives and utilizes waste energy from
the afterburner 12 of the inclined rotary reduction smelter (IRRS) process
as described in U.S. Pat. No. 4,758,268, issued July 19, 1988, (see FIG.
2), or from similar companion primary metallurgical processes capable of
supplying the principal source of waste energy needed to drive the
process. The invention may also stand alone by utilizing primary energy
sources such as natural gas, oil, producer gas, coal, other sources of
combustible materials, or combinations thereof.
The continuous feed shaft retort 10 as illustrated in FIG. 1 is a free
standing shaft type structure mounted within furnace or afterburner 12,
and is composed of refractory material which exhibits high physical
strength, high thermal conductivity, and relatively low permeability at
normal afterburner temperatures. Refractory material composed primarily of
silicon carbide is preferred for such a shaft retort.
The shaft retort 10 may be either cylindrical or rectangular, and is of
slightly conical or pyramidal shape, with cross sectional area of the
structure being less at the top and greater at the bottom. The cross
sectional area increases from top to bottom to limit or prevent the
wedging effect of burden material as it flows down through the shaft
retort, and to decrease the relative area through which the process
metallic gases must flow, thus slightly increasing the relative velocity
and/or pressure of the gases as they flow up through the vessel. This is
an important aspect of the invention as the principle of barometric
sealing is utilized to prevent the passage of metallic gases, carbon
monoxide, hydrogen, and carbon dioxide from the reduction and vaporization
zone 18 into the preheating and coking zone 14 of the continuous feed
shaft retort.
The cylindrical or conical shape is preferred for structural strength;
however, it is limited in diameter by the rate at which heat can be
effectively transferred to the center of the cross sectional area. The
rectangular or pyramidal shape may be utilized to allow better transfer of
heat to the center of the cross sectional area; however, such structures
do not enjoy the same structural strength as the cylindrical or conical
shaped vessel. The compromise between the two structures depends on the
working volume needed in the shaft retort to accommodate the desired
hourly through-put rate of process material.
The location of the metallic gas/process gas extraction duct (M/PGE duct)
20 is determined by the shape of the shaft retort utilized, the process
temperature, and the retention time required in the vertical retort to
complete the smelting process. In general, the M/PGE duct 20 is located
approximately mid-level of the heated section of the shaft retort 10. The
M/PGE duct intersects the shaft retort wall at an angle of approximately
45 degrees from the horizontal and is inclined upwardly through the
refractory lined wall of the afterburner 12 at the same angle. Immediately
outside the afterburner wall, the M/PGE duct 20 is turned about 90 degrees
to incline downwardly, then connects with a vertical duct 22, which in
turn connects with a splash condenser 24. The M/PGE ducts are constructed
of the same refractory material as the refractory lined shaft of the
retort and are insulated only exterior to the afterburner to prevent loss
of temperature of the gases as they pass into the splash condenser 24,
which is of conventional commercial design.
The top 26 of the shaft retort 10 extending through the roof of the
afterburner may be constructed of less expensive refractory material than
silicon carbide, as its principal function is only to contain the
preheated/dried greenball pellet feed for the continuous feed shaft retort
process and to collect and distribute the gaseous volatile matter
(primarily methane) which is driven off the greenball pellet in the
preheating zone 14 of the shaft retort. The external portion of the top 26
of the shaft retort is insulated to keep the temperature of the top gas
above the dew-point until the gas passes into down leg 28 of top gas
collecting duct 30. In the down leg of the uninsulated top gas duct, the
gas cools and moisture is precipitated, collected, and separated from the
methane gas by way of a water trap 32.
Some coal oils also precipitate in the cooler down leg and become trapped
with the precipitated water. The water/oil emulsion is piped to the
pelletizing circuit to be consumed in the pellet making process.
The methane gas collected from the top of the shaft retort is mixed with
spent process gas (from the splash condenser 24) which is primarily CO and
CO.sub.2 gas, with minor amounts of H.sub.2, H.sub.2 O, and ZnO: and the
combined secondary process gas contains a potential heating value of
between 500 and 600 BTU per cubic foot. The secondary process gas is
further scrubbed of moisture and solid particulate matter by passing
through a gas cooler 34 and fabric filter (bag house) 36 before being
compressed in compressor 38 and stored in a gas holder 40 for further use
as a primary combustion gas in the main burner 41 of the secondary
greenball pellet dryer 42. Excess secondary process gas is piped for use
as a primary combustion gas in the main burner of the primary greenball
pellet dryer of the IRRS process and/or as the primary combustion gas in
the main burner 44, and auxiliary burners 46 of the afterburner 12, or the
stand-alone furnace.
The secondary greenball pellet dryer is adapted to receive, screen, and dry
a secondary greenball pellet. The secondary greenball pellet is conveyed
through the high temperature section or drying zone 48 of the dryer 42 on
a high alloy wire mesh belt 50. High temperature products-of-combustion
gas produced by the main burner 41 of the dryer are mixed with tempering
air in the dryer combustion chamber to reach a process (drying) gas
temperature of 600.degree. C. before passing downward from the combustion
chamber through checkerwork refractory, which distributes the high
temperature drying gas evenly over the entire bed of pellets.
The secondary greenball dryer has two drying zones, as follows:
Preheating zone 52 --in which a drying gas of approximately 300.degree. C.
passes in an up-draft direction. The wet pellet bed acts to filter any
entrained dust particles from the drying zone gas; thus, only steam is
emitted from the dryer stack 54.
Drying zone 48 --in which a drying gas of approximately 600.degree. C.
passes in a down-draft direction. After passing down through the pellet
bed the drying gas is thereafter directed upwardly through the preheating
zone 52.
Waste gases emitted from the secondary greenball pellet dryer are composed
of CO.sub.2 and H.sub.2, and are devoid of particulate matter.
Pellet fines which pass through the high alloy wire mesh belt 50 are
collected via a screw conveyor 56 from the base of the dryer body and are
recycled to the secondary pelletizing circuit, shown in FIG. 3.
Dried, and preheated (about 300.degree. C.) secondary greenball pellets
pass from the dryer 42 directly into an insulated holding bin 60 on or at
the top 26 of the shaft retort 10. The bed of dry pellets in the holding
bin serves adequately as a barometric seal to prevent methane gas, which
is emitted from the top of the shaft retort, from passing into the dryer.
Hot methane gas emitted from the pellets in the preheating and/or coking
zone 14 of the shaft retort pass up through the pellet burden into the top
gas collecting duct 30. The gas pressure in the top gas collecting duct 30
is lower relative to the gas pressure in the top of the continuous feed
shaft retort; thus, the gas flows into the top gas collecting duct.
The dried and preheated secondary greenball pellet entering the top of the
continuous feed shaft retort is typically composed of the following
components:
______________________________________
TYPICAL GREENBALL PELLET ANALYSIS
Analysis Percent
______________________________________
TOTAL Fe 3.70
Fe.sub.2 O.sub.3 5.03
SiO.sub.2 2.49
Al.sub.2 O.sub.3 1.04
CaO 2.74
MgO 0.57
MnO 0.49
S 0.45
PbO 8.02
ZnO 43.13
CdO 0.13
Na.sub.2 O 2.46
K.sub.2 O 1.90
Fixed Carbon 11.15
Volatile Matter (CH.sub.4)
7.68
Other (Ash) Balance
______________________________________
The continuous feed shaft retort is divided into four principal process
zones as follows:
First, the preheating and/or coking zone 14 where the greenball pellet
enters at a temperature of approximately 300.degree. C. and is raised to
approximately 900.degree. C. before passing into the transition zone 16 in
the region where the M/PGE duct 20 connects to the shaft retort 10.
Methane gas contained in the admixed coal in the greenball pellet becomes
volatilized and is eliminated from the pellet when the pellet bed reaches
600.degree. C. The hot methane gas passes up through the pellet bed and
promotes heat transfer to the upper area of the bed before the gas exits
the shaft retort via the top gas collection duct 30. Coal selected for the
pelletizing process must be carefully selected to exhibit non-swelling
properties and may have caking, or slightly caking properties.
Additionally, the coal must have a relatively high ash fusion temperature
under reducing atmosphere. This is an important aspect of this invention,
as the greenball pellets are cured and coked in the preheating/coking zone
14 of the continuous feed shaft retort (where pellet integrity is very
important). The practice of previous inventions is to sinter or coke the
feed material external to the vertical retort. The temperature in the
pellet bed in the preheating/coking zone 14 is controlled by the
temperature within the afterburner 12 surrounding the shaft retort. The
principal source of heat inside the bed is transferred to the bed
primarily by radiation from the silicon carbide refractory of the shaft
retort and secondarily by conduction from pellet-to-pellet and convection
from gas-to-pellet. To accomplish adequate heat transfer through the shaft
retort refractory from the afterburner 12 to the pellet bed inside the
shaft retort, the temperature in the afterburner in the area of the
preheating/coking zone must be maintained between 1000.degree. and
850.degree. C. Ancillary shell burner/blowers 46 are employed tangentially
at various elevations on the afterburner shell 13 to control the
temperature inside the various reaction zones of the shaft retort.
Transition zone 16, in which the pellet bed temperature is raised to above
900.degree. C. as the body of the pellet bed moves down into an expanding
chamber, is defined as the region of the connection of the M/PGE duct 20
with the body of the shaft retort. The gas pressure inside the M/PGE duct
is negative relative to the gas pressure in the bed of the pellets in the
area of the transition zone. Thus, metallic vapor generated in the
reduction/vaporization zone 18 of the shaft retort flows from the bed into
the M/PGE duct and thence to the splash condenser 24 where the metallic
fumes are condensed and collected as liquid metal in the hearth of the
condenser.
Reduction and vaporization zone 18, in which the pellet bed is raised from
900.degree. C. to between 1050.degree. and 1150.degree. C. in a strongly
reducing atmosphere, is defined by the lower portion of the retort within
the afterburner. Metal oxides contained in the pellets are reduced to the
metallic state, and at the temperature and atmospheric conditions
employed, the non-ferrous metals largely change to the vapor (gaseous)
state and enter the process gas stream. Once in the gas stream, the metal
vapors pass out of the shaft retort via the M/PGE duct 20 from the
transition zone 16. The process gas stream includes CO and CO.sub.2, which
are generated as exemplified by the following equations:
2C+O.sub.2 =2CO
3Fe.sub.2 O.sub.3 +CO =2Fe.sub.3 O.sub.4 +CO.sub.2
Fe.sub.3 O.sub.4 +CO=3FeO+CO.sub.2
FeO+CO=Fe+CO.sub.2
ZnO(.sub.z)+CO=Zn(.sub.g)+CO.sub.2
PbO(.sub.z)+CO=Pb(.sub.g)+CO.sub.2
CdO(.sub.z)+CO=Cd(.sub.g)+CO.sub.2
Note: in the presence of excess carbon in the pellet, and with oxygen being
available only from the oxides of metals within the pellet, the ratio of
CO to CO.sub.2 evolving from the reduction process will be very high.
Based on the energy needed to raise the temperature of the secondary
greenball pellet feed to the continuous feed shaft retort from 300.degree.
C. to the reduction temperature of 1050.degree. C., and including the
energy needed to drive the reduction process, it is calculated that
approximately 7.times.10.sup.6 BTU/ton is needed to drive the total
process. As the secondary process gas emitted from the continuous feed
shaft retort 10 contains the potential heating value of approximately
6.89.times.10.sup.6 BTU/ton feed, the process is almost autogenous even
without the use of the waste energy from the after-burner of the companion
pyrometallurgical process.
The lowermost portion of the shaft retort is comprised of two principal
zones as follows: A spent residue excavation zone 64 and a solid residue
cooling or melting zone 66.
Spent residue excavation zone 64, includes a refractory insulated vibrating
or rotating table 68 capable of operating either under cooling or heating
conditions. The table operates in a refractory insulated chamber 70 which
is sealed from the atmosphere. Spent residue exits from the shaft retort
by gravity through a gap 72 between the bottom of the shaft retort and the
top surface of the discharge table. The gap between the shaft retort and
the discharge table allows passage of clinkers equal to minimum of
one-half the diameter of the shaft retort discharge area. Spent residue is
removed from the discharge table either by a stationary scraper, in the
case of a rotating table, or by the reversing action and angle-of-repose
of a vibrating table, or by the angle of the surface of the table relative
to the angle-of-repose of the spent residue.
The spent residue excavation zone is equipped to provide the input of
additional energy in the form of an oxy/fuel burner 74 which can be
operated either in oxidizing or reducing mode. The oxy/fuel burner
provides additional energy input into the shaft retort to increase the
transfer of high temperature energy into the pellet bed residing in the
reduction vaporization zone of the continuous feed shaft retort. Further,
by the nature of the oxy/fuel burner, additional internal energy input is
accomplished without the addition of nitrogen to the
products-of-combustion. The relatively low gas volume and high energy
input from the oxy-fuel burner greatly increases the daily productivity of
the continuous feed shaft retort process relative to the ratio of the
working volume of the shaft retort. The low gas volume emitted by the
oxy-fuel burner 74, as compared to the addition of air to the discharge
zone as is taught by the Handwerk et el patents, avoids the addition of
the diluting effect of nitrogen in the displacement gas in the shaft
retort; therefore, a relatively larger quantity of energy can be
transferred to the pellet bed in the reduction or vaporization zone.
The addition of combustion gases in the discharge area of the shaft retort,
whether from the addition of air to react with residue carbon contained in
the spent residue or from an oxy/fuel burner, must also be limited to
prevent the temperature generated in the pellet bed from exceeding the
fusion temperature of material in the bed. Uncontrolled fusion in the bed
would cause excessive clinkering or sintering of the bed and hinder or
prevent the free flow of the burden through the shaft retort. The oxy/fuel
burner can be controlled to limit the temperature of the
products-of-combustion to such level as to prevent fusion of the burden by
the ratio of natural gas volume used to the volume of oxygen employed in
the burner. Further, by operating the oxy/fuel burner in the spent residue
excavation zone in the reducing mode (natural gas rich) the excess natural
gas becomes reformed in endothermic reactions to form CO and H.sub. 2
reducing gases which act to improve and complete reduction of the metal
oxides which remain unreduced in the pellet burden in the lower part of
the shaft retort and in the spent residue excavation zone. The additional
reducing value introduced by using excess natural gas via the oxy/fuel
burner allows the feed material comprising the secondary greenball pellet
to employ less excess carbon in the form of coal, anthracite, or coke;
thus improving the overall economics of the continuous feed shaft retort
process relative to previously known processes.
Another source of gaseous fuel (natural gas and/or secondary process gas)
may be injected through the steel shaft 69 of the rotary table 68. One
purpose for such gaseous injection is to provide cooling for the steel
shaft itself while preheating the gas being injected. Such gaseous
injection through the rotary discharge table also acts to cool the
refractory at the point of the highest temperature burden material exiting
from the center of the shaft retort. By the injection of
products-of-combustion via the oxy/fuel burner and/or via the steel shaft
of the rotary discharge table, the temperature of the bed of spent residue
contained on the discharge table is maintained between 1050 and
1150.degree. C. At such temperatures, the spent residue is maintained
below the point of incipient fusion and well above adequate reduction
temperatures.
Solid residue cooling or melting zone 66 is shown in FIG. 4. Depending on
the level of lead retained in the spent residue exiting the spent residue
excavation zone and the end disposition of the spent residue, two methods
of final dispersal are employed as follows: In water quench 78, spent
residue is dropped directly from the spent residue excavation zone through
a feed tube into a water quench tank. The water quench tank is sealed from
the atmosphere by a water seal formed by the discharge feed tube extending
into the water quench tank below the level of water contained in the tank.
Cooled spent residue is extracted from the water quench tank by drag chain
80 or a screw conveyor. The recovered spent residue is recycled to the
primary pelletizing circuit of the IRRS process.
Alternatively, the discharge can be as shown in FIG. 4, in which the spent
residue exiting the spent residue excavation zone 64 is discharged into a
refractory insulated holding bin 84, which in turn meters the material
into a melting furnace 86 wherein the temperature of the material is
raised to above 1450.degree. C. and melted. In the melting furnace,
additional slag forming agents and small quantities of scrap iron (not
steel) may be added as needed to lower the liquidus temperature of the
iron and slag bath to the lowest economical potential. As all metals
remaining in the spent residue enter the melting furnace essentially free
of oxides, and the atmosphere in the furnace is maintained under reducing
conditions, very little, if any, reformation of metal oxides occur. Metals
such as iron and lead remaining in the spent residue collect in liquid
form in the hearth of the furnace. The collected liquid metals separate by
gravity and are decanted periodically at 88, and slag (low in FeO) will be
continuously discharged at slag notch 90. Gases formed in the melting
furnace are removed through duct 92 back into the spent residue extraction
zone for recycle through the continuous feed shaft retort process. The
gases contain CO, CO.sub.2, H.sub.2, plus metallic vapors of Zn, Pb, and
Cd. The melting furnace is of conventional commercial design.
The following summarizes the operation of the invented method:
The present invention employs greenball pellet as the process feed stock
instead of the sintered or coked feed stock required by previous vertical
retort processes. We utilize a carbon to metallic oxide balanced and
pelletized feed material which is cured and reduced in one continuous
operation, unlike known processes which use separate sintering or coking
furnaces to indurate the feed material prior to entering the vertical
retort. Separate sintering or coking steps are inherently subject to
emitting hazardous heavy metal fumes to the environment surrounding the
operating facility, which renders them dangerous to persons in the area.
By intimately blending carbonaceous reductants with the host secondary flue
dust (or other sources of metal bearing ores) in the form of greenball
pellets (in the size range of 1/4 to 3/4 inch diameter) with a
commercially available binding agent, our resulting greenball pellet has
excellent thermal shock resistance and has adequate physical strength for
direct placement into the high temperature atmosphere of the shaft retort
without suffering critical disintegration.
The greenball pellets of this invention, in addition to having the good
qualities listed, above also provide excellent bed permeability, high heat
transfer qualities, and superior reducibility due to significantly higher
internal pellet porosity (as compared to the more dense presintered, or
precoked agglomerates used by previous processes).
The invented system reprocesses secondary flue dust from sources of primary
metallurgical production, and technically the process is capable of
handling low as well as high metallic content (ferrous and non-ferrous)
feed material.
The invented system provides significant energy conservation relative to
previously known vertical retort processes and systems. Volatile matter,
essentially methane which is contained in the uncoked carbonaceous
reductant admixed and pelletized with host metal bearing flue dust, is
volatilized in the preheating and coking zone in the upper portion of the
shaft retort and exits the shaft retort via the top gas collecting duct.
The relatively pure methane gas is mixed with the CO/CO.sub.2 gas from the
liquid metal condensing system, and the mixed gas containing between 500
and 600 BTU per cubic foot is cleaned and recycled as a fuel for the
principal process burners.
The principal high temperature energy needed to drive the reduction process
in the continuous feed shaft retort may be supplied in total or in part by
waste energy contained in waste gases from the companion primary
metallurgical process.
The greenball pellet developed for use in the continuous feed shaft retort
process contains only about ten percent fixed carbon (FC), whereas
previous vertical retort inventions require more than twenty percent of
fixed carbon in the solid burden feed to the retort. By the practice of
this invention, the excess carbon or reducing agents needed to complete
the reduction processes in the shaft retort are supplied by the
reformation of natural gas and/or recycled secondary process gases as
described above.
This invention provides greatly improved gas-to-solids heat transfer within
the pellet bed inside the reduction/vaporization zone of the shaft retort
by injection of nitrogen free, low CO.sub.2, and high temperature
combustion gases produced by the oxy/fuel burner, located in the spent
residue excavation zone below the shaft retort.
The absence of nitrogen from the products-of-combustion emitted from the
oxy/fuel burner allows a much higher quality reducing and heat transfer
"displacement" gas to be introduced internally into the shaft retort
without excessively diluting the metal vapors contained in the hot process
gases which enter the liquid metal condensing system from the continuous
feed shaft retort.
The improvement in heat transfer allows a significant improvement in the
productivity of the shaft retort as less relative working volume (cubic
feet capacity) is needed per ton of material processed per unit of time.
This fact is quantified by a comparison of the relative volumes of gas and
energy content which can be injected into the vertical retort of previous
inventions with this invention as follows:
On an equal gas volume-to-gas volume basis, the injection of the gas volume
generated as the products-of-combustion (incomplete) from the oxy/fuel
burner of this invention will supply approximately one third more reducing
gas and three times more BTU (heat energy) than the injection of air as
practiced by previous inventions.
Because the combustion reactions mentioned above as "incomplete" are oxygen
deficient, the resulting actual flame temperature will be suppressed
sufficiently to prevent the material residing within the spent residue
excavation zone from exceeding the temperature of incipient fusion.
Due to the significant improvement in the ability of this system to
transfer heat to the center of the pellet bed inside the reduction and
vaporization zone of the shaft retort, the former limit of approximately
twelve to fifteen inches of internal width between heated wall surfaces
can be increased to between twenty-four to thirty inches - depending on
the exact temperature of the point of incipient fusion of the particular
material being processed. The relative improvement in working volume per
vertical foot of retort height is on the order of three to four.
In this invention, the transition zone provides an area in which the pellet
bed moves into an open area where gases in the pellet bed are allowed to
expand into a lower pressure area comprised of the metallic/process gas
extraction duct. By maintaining the bed temperature in the transition zone
above 900.degree. C., problems associated with "refluxing" of metal vapors
with prematurely condensed liquid metal and pellets within the pellet bed
in prior art vertical retorts are avoided.
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